Imaging lens

ABSTRACT

An imaging lens includes a first lens having positive refractive power; a second lens having negative refractive power; a third lens; a fourth lens; a fifth lens; and a sixth lens. The fifth lens is formed in a shape so that a surface thereof on the object side and a surface thereof on the image plane side have negative curvature radii. The sixth lens is formed in a shape so that a surface thereof on the object side and a surface thereof on the image plane side have positive curvature radii. The fourth to sixth lenses have specific Abbe&#39;s numbers to satisfy specific conditions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of a prior application Ser. No.14/483,224, filed on Sep. 11, 2014, pending, which is a continuationapplication of a prior application Ser. No. 13/963,488, filed on Aug. 9,2013, allowed and issued as U.S. Pat. No. 8,861,099.

BACKGROUND OF THE PRESENT INVENTION AND RELATED ART STATEMENT

The present invention relates to an imaging lens for forming an image ofan object on an imaging element such as a CCD sensor and a CMOS sensor.In particular, the present invention relates to an imaging lens suitableto be mounted in a relatively small camera such as a camera built in aportable device such as a cellular phone and a portable informationterminal, a digital still camera, a security camera, a vehicle onboardcamera, and a network camera.

In these years, in place of cellular phones that are intended mainly formaking phone calls, so-called “smartphones” have been more widely used,i.e., a cellular phones with such functions as those of portableinformation terminals (PDA) and/or personal computers. Since thesmartphones generally are highly functional as opposed to the cellularphones, it is possible to use images taken by a camera thereof invarious applications. For example, in case of the smartphone, while itis possible to use for printing and enjoying images taken as of itsintended use, it is also possible to use in additional uses such asprocessing images to be used in games or for makeup simulations, dressfitting simulations, and the others. Such uses of the images, which werenot conventionally common, are becoming increasingly popular every year.

Generally speaking, a product group of cellular phones and smartphonesis often composed of products with various specifications such as thosefor beginner users and those for advanced user. Among them, an imaginglens to be mounted in the cellular phone or the smartphone, which isdeveloped for advanced users, is required to have a high resolution lensconfiguration so as to be also applicable to a high pixel count imagingelement of these days. However, as the imaging lens to be mounted insmartphones used for the above-described usages, it is critical to be asmall size with a wide angle of view, that is, a wide angle, than havinga high resolution. Especially in these days, with advancements indownsizing and high performances of smartphones, an imaging lens hasbeen required to have smaller size and wider angle of view.

Accordingly, in case of cellular phones or smartphones, depending onsignificance in the product group, there is slight difference inspecifications of imaging lens for mounting in cellular phones andsmartphones. It is preferred to choose a most preferable lensconfiguration for each required specification, but in view of solvingproblems including shortening of the product development period and costreduction, a five-lens or six-lens configuration is desired. In a lensconfiguration composed of six lenses, since the number of lenses thatcompose an imaging lens is many, although it is slightly disadvantageousfor downsizing of the imaging lens, there is flexibility in designing,so that there is potential to attain satisfactory correction ofaberrations and downsizing of the imaging lens in a balanced manner. Asa lens configuration composed of six lenses, for example, the onedescribed in Patent Reference has been known.

Patent Reference: Japanese Patent Publication No. 2011-145315

The imaging lens described in Patent Reference includes a first lensthat is negative and has a shape of a meniscus lens directing a convexsurface thereof to an object side; a bonded lens composed of two lenses,positive and negative lenses; a fourth lens that is positive; and abonded lens composed of two lenses, positive and negative lenses.According to the imaging lens of Patent Reference, satisfying aconditional expression about curvature radii of an object-side surfaceand an image plane-side surface of the first lens and conditionalexpressions of the two bonded lenses, it is achievable to satisfactorilycorrect a distortion and a chromatic aberration.

According to the imaging lens described in Patent Reference, however,since a distance from the object-side surface of the first lens to animage plane of an imaging element is long, in order to mount the imaginglens in a small camera such as cellular phones and smartphones, it isnecessary to bend a light path with a prism or a mirror disposed betweenthe imaging lens and the image plane. Functions and Sizes of cellularphones and smartphones are higher and smaller every year, and the levelof downsizing required for the imaging lens is even higher than before.According to the lens configuration described in Patent Reference, it isdifficult to attain satisfactory aberration while attaining downsizingof the imaging lens to meet those requirements.

Here, such problem is not a problem specific to the imaging lens to bemounted in cellular phones and smartphones. Rather, it is a commonproblem even for an imaging lens to be mounted in a relatively smallcamera such as digital still cameras, portable information terminals,security cameras, onboard cameras, and network cameras.

An object of the present invention is to provide an imaging lens thatcan satisfactorily correct aberrations. In addition, a second object ofthe present invention is to provide an imaging lens that can attain bothdownsizing of the imaging lens and satisfactorily corrected aberrations.

Further objects and advantages of the present invention will be apparentfrom the following description of the present invention.

SUMMARY OF THE PRESENT INVENTION

In order to attain the objects described above, according to a firstaspect of the present invention, an imaging lens includes a first lenshaving positive refractive power; a second lens having negativerefractive power; a third lens having positive refractive power; afourth lens having positive refractive power; a fifth lens havingnegative refractive power; and a sixth lens having negative refractivepower, arranged in the order from an object side to an image plane side.The first lens has an object-side surface, a curvature radius of whichis positive. The second lens has an object-side surface and an imageplane-side surface, curvature radii of which are both positive. Thethird lens has an image plane-side surface, curvature radius of which isnegative. The fourth lens has an object-side surface and an imageplane-side surface, curvature radii of which are both negative. Thefifth lens has an object-side surface and an image plane-side surface,curvature radii of which are both negative. The sixth lens is formed ina shape so as to have negative refractive power near an optical axis andhave strong positive refractive power as it is close to the lensperiphery, and has an aspheric image plane-side surface having aninflexion point.

According to the first aspect of the present invention, when Abbe'snumbers of the first to the sixth lenses are νd1, νd2, νd3, νd4, νd5,and νd6, the imaging lens satisfies the following conditionalexpressions (1) to (6):45<νd1<75  (1)20<νd2<40  (2)45<νd3<75  (3)45<νd4<75  (4)20<νd5<40  (5)45<νd6<75  (6)

According to the first aspect of the present invention, in the imaginglens, two of the six lenses are made from a high-dispersion material. Inaddition, on an image plane side of the fifth lens made of thehigh-dispersion material, there is provided the sixth lens made of alow-dispersion material, and the sixth lens is formed such that theimage plane-side surface thereof is an aspheric shape having aninflexion point and so as to have negative refractive power near theoptical axis and have strong positive refractive power as it is close tothe periphery. For this reason, an off-axis light beam entered in theimaging lens goes through apositive-negative-positive-positive-negative-positive refractive powerpath from the first lens, and thereby an axial chromatic aberration aswell as an off-axis chromatic aberration of magnification issatisfactorily corrected. Here, since the refractive power arrangementof the first, the second, and the third lenses ispositive-negative-positive, the imaging lens of the present inventionhas a configuration that is advantageous for downsizing of the imaginglens while satisfactorily correcting aberrations.

When the imaging lens satisfies the conditional expressions (1) to (6),it is possible to satisfactorily correct the axial and off-axischromatic aberrations. Since Abbe's numbers of four out of six lensesare greater than the lower limit of “45”, chromatic aberrationsgenerated in those four lenses are effectively restrained and achromatic aberration of the whole lens system is suitably restrainedwithin satisfactory range. Moreover, having the Abbe's number of eachlens smaller than the upper limit of “75”, it is possible to restrainthe cost of the lens materials.

According to a second aspect of the present invention, when the wholelens system has a focal length f and a composite focal length of thefifth lens and the sixth lens is f56, the imaging lens having theabove-described configuration preferably satisfies the followingconditional expression (7):−1.0<f56/f<−0.3  (7)

When the imaging lens satisfies the conditional expression (7), it ispossible to define negative refractive powers of two lenses disposed onthe image plane side, and downsize the imaging lens while restraining anincident angle of a light beam emitted from the imaging lens to animaging element. In addition, it is also possible to restrain anastigmatism and a chromatic aberration within satisfactory ranges. Aswell known, in case of an imaging element of a CCD sensor, a CMOSsensor, or the like, there is set in advance a so-called chief ray angle(CRA), which is a range of an incident angle of a light beam that can betaken in the sensor. Restraining the incident angle of a light beamemitted from the imaging lens to an image plane within the CRA range, itis possible to suitably restrain generation of shading phenomenon, whichis a phenomenon of generation of an image with dark periphery.

When the value exceeds the upper limit of “−0.3” in the conditionalexpression (7), since a composite refractive power of the fifth lens andthe sixth lens is strong relative to the refractive power of the wholelens system, although it is advantageous for downsizing of the imaginglens, a back focal length is short, so that it is difficult to securespace for disposing an insert such as an infrared cut-off filter.Moreover, since the astigmatic difference increases, it is difficult toobtain satisfactory image-forming performance.

Furthermore, in this case, since a position of an exit pupil moves tothe image plane side, an incident angle of a light beam emitted from theimaging lens to the imaging element is large, so that it is difficult torestrain generation of the shading. On the other hand, when the value isbelow the lower limit of “−1.0”, since the composite refractive power ofthe fifth lens and the sixth lens is weak relative to the refractivepower of the whole lens system, the axial chromatic aberration isinsufficiently corrected (a focal position at a short wavelength movestowards the object side relative to a focal position at a referencewavelength). In addition, a chromatic aberration of magnification to anoff-axis light beam is insufficiently corrected (an image-forming pointat a short wavelength moves in a direction to get close to the opticalaxis relative to an image-forming point at a reference wavelength) inimage periphery, so that it is difficult to obtain satisfactoryimage-forming performance.

According to a third aspect of the present invention, when the wholelens system has the focal length f and the composite focal length of thefirst lens and the second lens is f12, the imaging lens having theabove-described configuration preferably satisfies the followingconditional expression (8):1.1<f12/f<3.0  (8)

When the imaging lens satisfies the conditional expression (8), it ispossible to satisfactorily correct a spherical aberration and a fieldcurvature, while attaining downsizing of the imaging lens. When thevalue exceeds the upper limit of “3.0”, the composite refractive powerof the first lens and the second lens is weak relative to the refractivepower of the whole lens system, so that the back focal length is longand it is difficult to attain downsizing of the imaging lens. Moreover,the image-forming surface curves to the image plane side and theastigmatic difference increases, so that it is difficult to obtainsatisfactory image-forming performance.

On the other hand, when the value is below the lower limit of “1.1”, thecomposite refractive power of the first lens and the second lens isstrong relative to the refractive power of the whole lens system,although it is advantageous for downsizing of the imaging lens, animage-forming surface curves to the object side and a sagittal imagesurface of the astigmatism tilts to the object side, so that it isdifficult to obtain satisfactory image-forming performance.

According to a fourth aspect of the present invention, when the firstlens has the focal length f1 and the second lens has the focal lengthf2, the imaging lens having the above-described configuration preferablysatisfies the following conditional expression (9):−0.8<f1/f2<−0.3  (9)

When the imaging lens satisfies the conditional expression (9), it ispossible to restrain the chromatic aberration, the astigmatism, and thefield curvature within satisfactory ranges, while attaining downsizingof the imaging lens. When the value exceeds the upper limit of “−0.3”,the first lens has strong refractive power relative to the second lens,and the axial chromatic aberration is insufficiently corrected. Inaddition, since periphery of the sagittal image surface of theastigmatism tilts to the image plane side and the astigmatic differenceand the field curvature increase, it is difficult to obtain satisfactoryimage-forming performance.

On the other hand, when the value is below the lower limit of “−0.8”,the first lens has weak refractive power relative to the second lens, sothat, although it is easy to restrain the axial chromatic aberrationwithin a satisfactory range, a position of the exit pupil moves to theobject side, and it is difficult to attain downsizing of the imaginglens. Moreover, periphery of the sagittal image surface of theastigmatism tilts to the image plane side and thereby the astigmaticdifference increases, so that it is difficult to obtain satisfactoryimage-forming performance.

According to a fifth aspect of the present invention, when the wholelens system has the focal length f and the third lens has the focallength f3, the imaging lens having the above-described configurationpreferably satisfies the following conditional expression (10):1.0<f3/f<5.0  (10)

When the imaging lens satisfies the conditional expression (10), it isachievable to downsize of the imaging lens and also restrain theastigmatism within satisfactory range, while restraining an incidentangle of a light beam emitted from the imaging lens to the imagingelement. When the value exceeds the upper limit of “5.0”, the third lenshas weak refractive power relative to the whole lens system and aposition of the exit pupil moves to the object side. Therefore, althoughit is easy to restrain the incident angle of a light beam emitted fromthe imaging lens to the imaging element within the range of CRA, theback focal length is long, so that it is difficult to attain downsizingof the imaging lens. Moreover, a sagittal image surface of theastigmatism tilts to the image plane side and the field curvatureincreases, so that it is difficult to obtain satisfactory image-formingperformance.

On the other hand, when the value is below the lower limit “1.0”, thethird lens has strong refractive power relative to that of the wholelens system, so that, although it is advantageous for downsizing of theimaging lens, it is difficult to secure the back focal length. Inaddition, it is difficult to restrain the incident angle of a light beamemitted from the imaging lens to the imaging element within the range ofCRA. Furthermore, in this case, since the sagittal surface of theastigmatism tilts to the object side, it is difficult to obtainsatisfactory image-forming performance.

According to a sixth aspect of the present invention, when the fourthlens has a focal length f4 and the fifth lens has a focal length f5, theimaging lens having the above-described configuration preferablysatisfies the following conditional expression (11):−0.4<f4/f5<−0.1  (11)

When the imaging lens satisfies the conditional expression (11), it ispossible to restrain a field curvature and an astigmatism withinsatisfactory range, while restraining an incident angle of a light beamemitted from the imaging lens to the imaging element. When the valueexceeds the upper limit of “−0.1”, since the fourth lens has strongrefractive power relative to the fifth lens, although it is advantageousfor downsizing of the imaging lens, an axial chromatic aberration isinsufficiently corrected. In addition, a sagittal image surface of theastigmatism tilts to the object side and the astigmatic differenceincreases, it is difficult to obtain satisfactory image-formingperformance.

On the other hand, when the value is below the lower limit of “−0.4”,the fourth lens has weak refractive power relative to the fifth lens,although it is advantageous for correction of a chromatic aberration,the image-forming surface curves to the image plane side, so that it isdifficult to obtain satisfactory image-forming performance. In addition,since the back focal length is long, it is difficult to attaindownsizing of the imaging lens. Furthermore, in this case, a position ofthe exit pupil moves towards the image plane side, so that it isdifficult to restrain the incident angle of a light beam emitted fromthe imaging lens to the imaging element within the range of CRA.

According to the imaging lens of the present invention, it is possibleto provide an imaging lens with satisfactorily corrected aberrations. Inaddition, it is possible to provide a small-sized imaging lens that isespecially suitable for mounting in a small-sized camera, while havinghigh resolution with satisfactorily corrected aberrations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view of a schematic configuration of an imaginglens in Numerical Data Example 1 according to an embodiment of thepresent invention;

FIG. 2 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 1;

FIG. 3 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 1;

FIG. 4 shows a sectional view of a schematic configuration of an imaginglens in Numerical Data Example 2 according to the embodiment of thepresent invention;

FIG. 5 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 4;

FIG. 6 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 4;

FIG. 7 shows a sectional view of a schematic configuration of an imaginglens in Numerical Data Example 3 according to the embodiment of thepresent invention;

FIG. 8 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 7;

FIG. 9 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 7;

FIG. 10 shows a sectional view of a schematic configuration of animaging lens in Numerical Data Example 4 according to the embodiment ofthe present invention;

FIG. 11 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 10;

FIG. 12 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 10;

FIG. 13 shows a sectional view of a schematic configuration of animaging lens in Numerical Data Example 5 according to the embodiment ofthe present invention;

FIG. 14 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 13; and

FIG. 15 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 13.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereunder, referring to the accompanying drawings, embodiments of thepresent invention will be fully described.

FIGS. 1, 4, 7, 10, and 13 are schematic sectional views of imaginglenses in Numerical Data Examples 1 to 5 according to the embodiment,respectively. Since a basic lens configuration is the same among thoseNumerical Data Examples, the lens configuration of the embodiment willbe described with reference to the illustrative sectional view ofNumerical Data Example 1.

As shown in FIG. 1, the imaging lens of the embodiment includes anaperture stop ST; a first lens L1 having positive refractive power; asecond lens L2 having negative refractive power; a third lens L3 havingpositive refractive power; a fourth lens L4 having positive refractivepower; a fifth lens L5 having negative refractive power; and a sixthlens L6 having negative refractive power, arranged in the order from anobject side to an image plane side. A filter 10 may be provided betweenthe sixth lens L6 and an image plane IM of an imaging element. Thefilter 10 may be optionally omitted.

According to the imaging lens having the above-described configuration,the first lens L1 is formed in a shape such that a curvature radius r1of an object-side surface thereof and a curvature radius r2 of an imageplane-side surface are both positive, i.e., a shape of a meniscus lensdirecting a convex surface thereof to an object side near an opticalaxis X. The shape of the first lens L1 is not limited to the one inNumerical Data Example 1. The shape of the first lens L1 can be any aslong as the curvature radius r1 of the object-side surface thereof ispositive and can be a formed in a shape such that the curvature radiusr2 of the image plane-side surface is negative, i.e., a shape of abiconvex lens near the optical axis X.

The second lens L2 is formed in a shape such that a curvature radius r3of an object-side surface thereof and a curvature radius r4 of an imageplane-side surface are both positive, i.e., a shape of a meniscus lensdirecting a convex surface thereof to the object side near the opticalaxis X.

The third lens L3 is formed in a shape such that a curvature radius r5of an object-side surface thereof and a curvature radius r6 of an imageplane-side surface are both negative, i.e., a shape of a meniscus lensdirecting a concave surface thereof to the object side near the opticalaxis X. The shape of the third lens L3 is not limited to the one inNumerical Data Example 1. The shape of the third lens L3 can be any aslong as the curvature radius r6 of the image plane-side surface thereofis negative, and can be a formed in a shape such that the curvatureradius r5 of the object-side surface is positive, i.e., a shape of abiconvex lens near the optical axis X. Numerical Data Examples 2 and 3are examples, in which the shape of the third lens L3 is a biconvex lensnear the optical axis X.

The fourth lens L4 is formed in a shape such that a curvature radius r7of an object-side surface thereof and a curvature radius r8 of an imageplane-side surface are both negative, i.e., a shape of a meniscus lensdirecting a concave surface thereof to the object side near the opticalaxis X.

The fifth lens L5 is formed in a shape such that a curvature radius r9of an object-side surface thereof and a curvature radius r10 of an imageplane-side surface are both negative, i.e., a shape of a meniscus lensdirecting a concave surface thereof to the object side near the opticalaxis X.

The sixth lens L6 is formed in a shape such that a curvature radius r11of an object-side surface thereof and a curvature radius r12 of an imageplane-side surface are both positive, i.e., a shape of a meniscus lensdirecting a convex surface thereof to the object side near the opticalaxis X. In addition, the object-side surface and the image plane-sidesurface thereof are formed as aspheric shapes having an inflexion point.More specifically, the sixth lens L6 is formed in a shape so as to havenegative refractive power near the optical axis X and have strongpositive refractive power as it is close to the lens periphery. Withsuch shape of the sixth lens L6, an off-axis light beam entered in theimaging lens goes through apositive-negative-positive-positive-negative-positive refractive powerpath from the first lens L1, so that an axial chromatic aberration aswell as an off-axis chromatic aberration of magnification issatisfactorily corrected. Moreover, an incident angle of a light beamemitted from the imaging lens to the image plane IM is suitablyrestrained within a range of Chief Ray Angle (CRA).

Here, according to the embodiment, the sixth lens L6 is formed as anaspheric shape such that both the object-side surface and the imageplane-side surface thereof have an inflexion point, but is notnecessarily formed as an aspheric shape such that those surfaces have aninflexion point. Even when only one of those surfaces is formed as anaspheric surface having an inflexion point, it is still possible to formthe sixth lens L6 in a shape so as to have negative refractive powernear the optical axis X and have strong positive refractive power as itis close to the lens periphery. Whether both surfaces of the six lens L6have an inflexion point or only one surface thereof has an inflexionpoint often depends on optical performances required for the imaginglens, but in view of restraining the incident angle of a light beamemitted from the imaging lens to the image plane IM within the range ofCRA, it is preferred to have an inflexion point at least on the imageplane-side surface of the sixth lens L6.

The imaging lens of the embodiment satisfies the following conditionalexpressions (1) to (11):45<νd1<75  (1)20<νd2<40  (2)45<νd3<75  (3)45<νd4<75  (4)20<νd5<40  (5)45<νd6<75  (6)−1.0<f56/f<−0.3  (7)1.1<f12/f<3.0  (8)−0.8<f1/f2<−0.3  (9)1.0<f3/f<5.0  (10)−0.4<f4/f5<−0.1  (11)

In the above conditional expressions:

f: Focal length of the whole lens system

f1: Focal length of a first lens L1

f2: Focal length of a second lens L2

f3: Focal length of a third lens L3

f4: Focal length of a fourth lens L4

f5: Focal length of a fifth lens L5

f12: Composite focal length of the first lens L1 and the second lens L2

f56: Composite focal length of the fifth lens L5 and the sixth lens L6

νd1: Abbe's number of the first lens L1

νd2: Abbe's number of the second lens L2

νd3: Abbe's number of the third lens L3

νd4: Abbe's number of the fourth lens L4

νd5: Abbe's number of the fifth lens L5

νd6: Abbe's number of the sixth lens L6

Here, it is not necessary to satisfy all of the conditional expressions,and it is achievable to obtain an effect corresponding to the respectiveconditional expression when any single one of the conditionalexpressions is individually satisfied.

In the embodiment, all lens surfaces of the first lens L1 to the sixthlens L6 are formed as an aspheric surface. When the aspheric surfacesapplied to the lens surfaces have an axis Z in a direction of theoptical axis X, a height H in a direction perpendicular to the opticalaxis X, a conical coefficient k, and aspheric coefficients A₄, A₆, A₈,and A₁₀, a shape of the aspheric surfaces of the lens surfaces isexpressed as follows:

$\begin{matrix}{Z = {\frac{\frac{H^{2}}{R}}{1 + \sqrt{1 - {\left( {k + 1} \right)\frac{H^{2}}{R^{2}}}}} + {A_{4}H^{4}} + {A_{6}H^{6}} + {A_{8}H^{8}} + {A_{10}H^{10}}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

Next, Numerical Data Examples of the imaging lens of the embodiment willbe described. In each Numerical Data Example, f represents a focallength of the whole lens system, Fno represents an F number, and ωrepresents a half angle of view, respectively. In addition, i representsa surface number counted from the object side, r represents a curvatureradius, d represents a distance between lens surfaces (surface spacing)on the optical axis, nd represents a refractive index, and νd representsAbbe's number, respectively. Here, aspheric surfaces are indicated withsurface numbers i affixed with * (asterisk).

Numerical Data Example 1

Basic Data are shown below.

f = 3.66 mm, Fno = 2.2, ω = 38.0° Unit: mm Surface Data Surface Number ir d nd νd (Object) ∞ ∞ 1* (Stop) 1.664 0.490 1.5346 56.2(=νd1) 2* 55.9730.010 3* 2.584 0.256 1.6355 23.9(=νd2) 4* 1.736 0.357 5* −6.143 0.5571.5438 55.8(=νd3) 6* −3.031 0.310 7* −3.571 0.603 1.5438 55.8(=νd4) 8*−1.087 0.048 9* −2.172 0.297 1.6355 23.9(=νd5) 10*  −4.137 0.051 11* 5.706 0.483 1.5346 56.2(=νd6) 12*  1.014 0.400 13  ∞ 0.300 1.5168 64.214  ∞ 0.374 (Image ∞ plane) f1 = 3.18 mm f2 = −9.35 mm f3 = 10.31 mm f4= 2.64 mm f5 = −7.57 mm f6 = −2.38 mm f12 = 4.23 mm f56 = −1.68 mmAspheric Surface Data First Surface k = −3.931E−01, A₄ = 2.248E−02, A₆ =2.869E−03, A₈ = 2.071E−03, A₁₀ = −7.364E−03 Second Surface k = 0.000, A₄= −1.757E−02, A₆ = 5.509E−02, A₈ = −2.150E−02, A₁₀ = −5.117E−02 ThirdSurface k = −7.847, A₄ = −5.725E−02, A₆ = 1.100E−01, A₈ = −3.550E−02,A₁₀ = −6.239E−02 Fourth Surface k = 1.488, A₄ = −1.248E−01, A₆ =4.036E−02, A₈ = 4.550E−02, A₁₀ = −5.377E−02 Fifth Surface k = 0.000, A₄= −5.070E−02, A₆ = −2.739E−02, A₈ = 1.624E−02, A₁₀ = 9.241E−02 SixthSurface k = 9.587E−01, A₄ = −3.945E−02, A₆ = 2.979E−02, A₈ = −5.416E−02,A₁₀ = 5.463E−02 Seventh Surface k = 1.318, A₄ = −1.363E−02, A₆ =4.633E−03, A₈ = −4.068E−04, A₁₀ = −1.838E−03 Eighth Surface k = −1.174,A₄ = 1.264E−01, A₆ = −9.436E−02, A₈ = 4.639E−02, A₁₀ = −8.842E−03 NinthSurface k = 4.381E−01, A₄ = −4.305E−02, A₆ = 1.826E−02, A₈ = −7.458E−04,A₁₀ = −1.245E−03 Tenth Surface k = −2.530, A₄ = −2.632E−03, A₆ =−1.464E−03, A₈ = −1.408E−04, A₁₀ = 2.518E−04 Eleventh Surface k =−4.490E+02, A₄ = −4.139E−02, A₆ = 6.768E−03, A₈ = 2.909E−04, A₁₀ =−7.624E−05 Twelfth Surface k = −6.909, A₄ = −4.366E−02, A₆ = 7.908E−03,A₈ = −1.354E−03, A₁₀ = 7.761E−05

The values of the respective conditional expressions are as follows:f56/f=−0.46f12/f=1.16f1/f2=−0.34f3/f=2.82f4/f5=−0.35

Accordingly, the imaging lens of Numerical Data Example 1 satisfies theabove-described conditional expressions. A distance on the optical axisX from the object-side surface of the first lens L1 to the image planeIM (length in air) is 4.43 mm, and downsizing of the imaging lens isattained.

FIG. 2 shows a lateral aberration that corresponds to an image heightratio H, which is divided into a tangential direction and a sagittaldirection (which is the same in FIGS. 5, 8, 11, and 14). Furthermore,FIG. 3 shows a spherical aberration (mm), astigmatism (mm), and adistortion (%), respectively. In the aberration diagrams, for thelateral aberration diagrams and spherical aberration diagrams,aberrations at each wavelength, i.e., a g line (435.84 nm), an e line(546.07 nm), and a C line (656.27 nm) are indicated. In the astigmatismdiagram, an aberration on a sagittal image surface S and an aberrationon a tangential image surface T are respectively indicated (which arethe same in FIGS. 6, 9, 12, and 15). As shown in FIGS. 2 and 3,according to the imaging lens of Numerical Data Example 1, theaberrations are satisfactorily corrected.

Numerical Data Example 2

Basic Data are shown below.

f = 4.20 mm, Fno = 2.4, ω = 34.2° Unit: mm Surface Data Surface Number ir d nd νd (Object) ∞ ∞ 1* (Stop) 1.678 0.453 1.5346 56.2(=νd1) 2* 15.1900.036 3* 3.693 0.361 1.6355 23.9(=νd2) 4* 1.585 0.325 5* 212.104 0.3171.5438 55.8(=νd3) 6* −3.973 0.530 7* −2.088 0.630 1.5438 55.8(=νd4) 8*−0.941 0.030 9* −2.086 0.295 1.6355 23.9(=νd5) 10*  −2.654 0.062 11* 4.257 0.506 1.5346 56.2(=νd6) 12*  1.097 0.500 13  ∞ 0.300 1.5168 64.214  ∞ 0.900 (Image ∞ plane) f1 = 3.47 mm f2 = −4.64 mm f3 = 7.15 mm f4 =2.63 mm f5 = −19.07 mm f6 = −2.92 mm f12 = 8.29 mm f56 = −2.37 mmAspheric Surface Data First Surface k = −4.341E−01, A₄ = 2.636E−02, A₆ =3.346E−02, A₈ = −5.451E−02, A₁₀ = 5.353E−02 Second Surface k = 0.000, A₄= −3.750E−03, A₆ = 7.392E−02, A₈ = −1.196E−02, A₁₀ = −1.803E−02 ThirdSurface k = −1.602E+01, A₄ = −6.074E−02, A₆ = 1.095E−01, A₈ =−2.518E−02, A₁₀ = −4.920E−02 Fourth Surface k = 1.269, A₄ = −1.380E−01,A₆ = 3.979E−02, A₈ = 3.077E−02, A₁₀ = −7.969E−02 Fifth Surface k =0.000, A₄ = −4.841E−02, A₆ = −2.391E−02, A₈ = −7.584E−03, A₁₀ =7.198E−02 Sixth Surface k = 2.362, A₄ = −2.634E−02, A₆ = 1.985E−02, A₈ =−5.710E−02 A₁₀ = 6.707E−02 Seventh Surface k = −2.318, A₄ = −4.166E−03,A₆ = 2.785E−03, A₈ = 5.737E−03, A₁₀ = −1.015E−04 Eighth Surface k =−1.141, A₄ = 1.305E−01, A₆ = −8.927E−02, A₈ = 4.830E−02, A₁₀ =−9.394E−03 Ninth Surface k = 3.883E−01, A₄ = −1.031E−02, A₆ = 2.401E−02,A₈ = −1.775E−03, A₁₀ = 3.107E−04 Tenth Surface k = 6.565E−02, A₄ =−6.384E−03, A₆ = 1.213E−03, A₈ = 3.863E−04, A₁₀ = 2.589E−04 EleventhSurface k = −4.031E+02, A₄ = −3.950E−02, A₆ = 5.345E−03, A₈ = 2.809E−04,A₁₀ = −2.179E−05 Twelfth Surface k = −8.583, A₄ = −4.045E−02, A₆ =8.872E−03, A₈ = −1.129E−03, A₁₀ = 5.618E−05

The values of the respective conditional expressions are as follows:f56/f=−0.56f12/f=1.97f1/f2=−0.75f3/f=1.70f4/f5=−0.14

Accordingly, the imaging lens of Numerical Data Example 2 satisfies theabove-described conditional expressions. A distance on the optical axisX from the object-side surface of the first lens L1 to the image planeIM (length in air) is 5.14 mm, and downsizing of the imaging lens isattained.

FIG. 5 shows the lateral aberration that corresponds to the image heightratio H of the imaging lens, and FIG. 6 shows a spherical aberration(mm), astigmatism (mm), and a distortion (%), respectively. As shown inFIGS. 5 and 6, according to the imaging lens of Numerical Data Example2, the aberrations are also satisfactorily corrected.

Numerical Data Example 3

Basic Data are shown below.

f = 3.59 mm, Fno = 2.2, ω = 38.5° Unit: mm Surface Data Surface Number ir d nd νd (Object) ∞ ∞ 1* (Stop) 1.684 0.431 1.5346 56.2(=νd1) 2* 12.3740.030 3* 3.750 0.270 1.6355 23.9(=νd2) 4* 1.603 0.268 5* 32.956 0.3851.5438 55.8(=νd3) 6* −3.418 0.486 7* −2.285 0.758 1.5438 55.8(=νd4) 8*−0.929 0.026 9* −2.055 0.278 1.6355 23.9(=νd5) 10*  −2.587 0.055 11* 3.888 0.526 1.5346 56.2(=νd6) 12*  1.071 0.500 13  ∞ 0.300 1.5168 64.214  ∞ 0.715 (Image ∞ plane) f1 = 3.58 mm f2 = −4.59 mm f3 = 5.69 mm f4 =2.39 mm f5 = −19.59 mm f6 = −2.95 mm f12 = 9.89 mm f56 = −2.39 mmAspheric Surface Data First Surface k = −4.846E−01, A₄ = 3.116E−02, A₆ =4.185E−03, A₈ = −1.529E−02, A₁₀ = 4.070E−02 Second Surface k = 0.000, A₄= −2.047E−03, A₆ = 7.998E−02, A₈ = −1.036E−02, A₁₀ = −1.640E−02 ThirdSurface k = −1.746E+01, A₄ = −5.647E−02, A₆ = 1.065E−01, A₈ =−2.790E−02, A₁₀ = −4.948E−02 Fourth Surface k = 1.240, A₄ = −1.414E−01,A₆ = 3.893E−02, A₈ = 2.920E−02, A₁₀ = −7.696E−02 Fifth Surface k =0.000, A₄ = −5.042E−02, A₆ = −2.374E−02, A₈ = −7.017E−03, A₁₀ =7.221E−02 Sixth Surface k = 1.839, A₄ = −1.899E−02, A₆ = 1.695E−02, A₈ =−6.350E−02, A₁₀ = 6.563E−02 Seventh Surface k = −2.511, A₄ = −4.250E−03,A₆ = 3.164E−03, A₈ = 5.737E−03, A₁₀ = −1.593E−04 Eighth Surface k =−1.122, A₄ = 1.245E−01, A₆ = −9.095E−02, A₈ = 4.839E−02, A₁₀ =−9.139E−03 Ninth Surface k = 3.437E−01, A₄ = −1.093E−02, A₆ = 2.490E−02,A₈ = −1.671E−03, A₁₀ = 3.762E−04 Tenth Surface k = 1.564E−02, A₄ =−4.726E−03, A₆ = 1.804E−03, A₈ = 4.641E−04, A₁₀ = 2.964E−04 EleventhSurface k = −4.916E+02, A₄ = −3.864E−02, A₆ = 5.309E−03, A₈ = 2.892E−04,A₁₀ = −1.734E−05 Twelfth Surface k = −9.610, A₄ = −3.680E−02, A₆ =8.626E−03, A₈ = −1.122E−03, A₁₀ = 5.909E−05

The values of the respective conditional expressions are as follows:f56/f=−0.67f12/f=2.75f1/f2=−0.78f3/f=1.58f4/f5=−0.12

Accordingly, the imaging lens of Numerical Data Example 3 satisfies theabove-described conditional expressions. A distance on the optical axisX from the object-side surface of the first lens L1 to the image planeIM (length in air) is 4.93 mm, and downsizing of the imaging lens isattained.

FIG. 8 shows the lateral aberration that corresponds to the image heightratio H of the imaging lens, and FIG. 9 shows a spherical aberration(mm), an astigmatism (mm), and a distortion (%), respectively. As shownin FIGS. 8 and 9, according to the imaging lens of Numerical DataExample 3, the aberrations are satisfactorily corrected.

Numerical Data Example 4

Basic Data are shown below.

f = 3.07 mm, Fno = 2.6, ω = 42.9° Unit: mm Surface Data Surface Number ir d nd νd (Object) ∞ ∞ 1* (Stop) 1.662 0.381 1.5346 56.2(=νd1) 2* 41.7070.022 3* 2.729 0.254 1.6355 23.9(=νd2) 4* 1.756 0.300 5* −9.263 0.3921.5438 55.8(=νd3) 6* −4.231 0.236 7* −2.867 0.762 1.5438 55.8(=νd4) 8*−1.006 0.045 9* −1.962 0.300 1.6355 23.9(=νd5) 10*  −2.495 0.052 11* 3.965 0.472 1.5346 56.2(=νd6) 12*  1.236 0.500 13  ∞ 0.300 1.5168 64.214  ∞ 0.365 (Image ∞ plane) f1 = 3.21 mm f2 = −8.54 mm f3 = 13.88 mm f4= 2.48 mm f5 = −18.34 mm f6 = −3.56 mm f12 = 4.51 mm f56 = −2.78 mmAspheric Surface Data First Surface k = −6.287E−01, A₄ = 1.553E−02, A₆ =−3.064E−02, A₈ = −2.019E−02, A₁₀ = 4.296E−02 Second Surface k = 0.000,A₄ = −4.017E−02, A₆ = 5.553E−02, A₈ = −1.564E−01, A₁₀ = −1.848E−01 ThirdSurface k = −8.206, A₄ = −6.487E−02, A₆ = 1.051E−01, A₈ = −4.760E−02,A₁₀ = −2.197E−01 Fourth Surface k = 1.613, A₄ = −8.779E−02, A₆ =1.183E−02, A₈ = 3.013E−02, A₁₀ = −3.821E−02 Fifth Surface k = 0.000, A₄= −7.345E−02, A₆ = −4.623E−02, A₈ = 4.608E−02, A₁₀ = 1.440E−01 SixthSurface k = 9.088E−01, A₄ = −4.230E−02, A₆ = 4.671E−02, A₈ = −5.657E−02,A₁₀ = 5.686E−02 Seventh Surface k = −7.126, A₄ = 1.410E−02, A₆ =−3.512E−04, A₈ = 8.811E−03, A₁₀ = −5.760E−03 Eighth Surface k =−9.449E−01, A₄ = 1.018E−01, A₆ = −7.723E−02, A₈ = 4.907E−02, A₁₀ =−8.899E−03 Ninth Surface k = 5.314E−01, A₄ = −2.140E−02, A₆ = 1.063E−02,A₈ = 2.237E−03, A₁₀ = 2.170E−04 Tenth Surface k = −1.043, A₄ =1.871E−03, A₆ = 4.767E−04, A₈ = −4.322E−04, A₁₀ = 1.879E−04 EleventhSurface k = −1.694E+01, A₄ = −4.537E−02, A₆ = 6.303E−03, A₈ = 2.992E−04,A₁₀ = −6.298E−05 Twelfth Surface k = −5.062, A₄ = −4.149E−02, A₆ =8.943E−03, A₈ = −1.296E−03, A₁₀ = 6.832E−05

The values of the respective conditional expressions are as follows:f56/f=−0.91f12/f=1.47f1/f2=−0.38f3/f=4.52f4/f5=−0.14

Accordingly, the imaging lens of Numerical Data Example 4 satisfies theabove-described conditional expressions. A distance on the optical axisX from the object-side surface of the first lens L1 to the image planeIM (length in air) is 4.28 mm, and downsizing of the imaging lens isattained.

FIG. 11 shows the lateral aberration that corresponds to the imageheight ratio H of the imaging lens, and FIG. 12 shows a sphericalaberration (mm), an astigmatism (mm), and a distortion (%),respectively. As shown in FIGS. 11 and 12, according to the imaging lensof Numerical Data Example 4, the aberrations are also satisfactorilycorrected.

Numerical Data Example 5

Basic Data are shown below.

f = 3.88 mm, Fno = 2.4, ω = 36.4° Unit: mm Surface Data Surface Number ir d nd νd (Object) ∞ ∞ 1* (Stop) 1.655 0.446 1.5346 56.2(=νd1) 2* 46.1960.020 3* 3.129 0.262 1.6355 23.9(=νd2) 4* 1.657 0.315 5* −523.041 0.3961.5438 55.8(=νd3) 6* −6.320 0.494 7* −2.145 0.729 1.5438 55.8(=νd4) 8*−0.898 0.038 9* −2.170 0.329 1.6355 23.9(=νd5) 10*  −2.779 0.092 11* 7.749 0.514 1.5346 56.2(=νd6) 12*  1.096 0.500 13  ∞ 0.300 1.5168 64.214  ∞ 0.514 (Image ∞ plane) f1 = 3.19 mm f2 = −5.90 mm f3 = 11.71 mm f4= 2.35 mm f5 = −19.58 mm f6 = −2.46 mm f12 = 5.53 mm f56 = −2.03 mmAspheric Surface Data First Surface k = −5.336E−01, A₄ = 1.656E−02, A₆ =6.433E−03, A₈ = 1.373E−02, A₁₀ = 1.355E−03 Second Surface k = 0.000, A₄= −8.518E−03, A₆ = 5.823E−02, A₈ = −6.831E−03, A₁₀ = −1.486E−02 ThirdSurface k = −9.815, A₄ = −5.808E−02, A₆ = 1.144E−01, A₈ = −2.665E−02,A₁₀ = −4.342E−02 Fourth Surface k = 1.423, A₄ = −1.371E−01, A₆ =4.496E−02, A₈ = 4.462E−02, A₁₀ = −7.843E−02 Fifth Surface k = 0.000, A₄= −4.419E−02, A₆ = −2.949E−02, A₈ = 1.409E−03, A₁₀ = 7.812E−02 SixthSurface k = 1.284, A₄ = −3.032E−02, A₆ = 2.719E−02, A₈ = −5.502E−02, A₁₀= 5.765E−02 Seventh Surface k = −1.532, A₄ = −2.683E−03, A₆ = 5.644E−03,A₈ = 4.117E−03, A₁₀ = 1.675E−03 Eighth Surface k = −1.164, A₄ =1.251E−01, A₆ = −9.504E−02, A₈ = 4.694E−02, A₁₀ = −8.551E−03 NinthSurface k = 1.668E−01, A₄ = −2.860E−02, A₆ = 1.693E−02, A₈ = −3.905E−03,A₁₀ = −1.162E−03 Tenth Surface k = −1.574E−01, A₄ = −8.418E−03, A₆ =2.701E−04, A₈ = 4.522E−04, A₁₀ = 2.969E−04 Eleventh Surface k =−1.573E+03, A₄ = −4.054E−02, A₆ = 6.479E−03, A₈ = 2.626E−04, A₁₀ =−7.078E−05 Twelfth Surface k = −8.511, A₄ = −3.726E−02, A₆ = 8.190E−03,A₈ = −1.343E−03, A₁₀ = 6.621E−05

The values of the respective conditional expressions are as follows:f56/f=−0.52f12/f=1.43f1/f2=−0.54f3/f=3.02f4/f5=−0.12

Accordingly, the imaging lens of Numerical Data Example 5 satisfies theabove-described conditional expressions. A distance on the optical axisX from the object-side surface of the first lens L1 to the image planeIM (length in air) is 4.85 mm, and downsizing of the imaging lens isattained.

FIG. 14 shows the lateral aberration that corresponds to the imageheight ratio H of the imaging lens, and FIG. 15 shows a sphericalaberration (mm), an astigmatism (mm), and a distortion (%),respectively. As shown in FIGS. 14 and 15, according to the imaging lensof Numerical Data Example 5, the aberrations are satisfactorilycorrected.

As described above, according to the imaging lens of the embodiment, itis possible to attain an angle of view (2ω) of 80° or greater. Here,angles of view of the imaging lenses in Numerical Data Examples 1 to 5are as wide as 68.4° to 85.8°. According to the imaging lens of theembodiment, it is possible to take an image in wider range than the onethat can be taken by a conventional imaging lens.

Furthermore, in order to improve camera performances, a high pixel countimaging element has been frequently used in combination with an imaginglens. In case of those high pixel-count imaging elements, since alight-receiving area of each pixel is smaller, an image taken tends tobe dark. To solve this problem, there is a method to improvelight-receiving sensitivity using an electrical circuit. However, sincea noise component, which does not directly contribute to imageformation, is also amplified, when the light-receiving sensitivityincreases, it is necessary to provide another circuit for reducingnoise. According to the imaging lenses of Numerical Data Examples 1 to5, the Fno is as small as 2.2 to 2.6. According to the imaging lens ofthe embodiment, it is possible to obtain sufficiently bright imagewithout providing the above-described electric circuit.

Accordingly, when the imaging lens of the embodiment is mounted in animage-forming optical system, such as built-in cameras of cellularphones, portable information terminals, and smartphones, digital stillcameras, security cameras, onboard cameras, and network cameras, it isachievable to have high functionality and downsize the cameras.

The present invention can be applied to imaging lenses to be mounted inrelatively small cameras, such as built-in cameras of portables devicesincluding cellular phones, smartphones, and portable informationterminals, digital still cameras, security cameras, onboard cameras, andnetwork cameras.

The disclosure of Japanese Patent Application No. 2012-180834, filed onAug. 17, 2012, is incorporated in the application by reference.

While the invention has been explained with reference to the specificembodiments of the invention, the explanation is illustrative and theinvention is limited only by the appended claims.

What is claimed is:
 1. An imaging lens comprising: a first lens havingpositive refractive power; a second lens having negative refractivepower; a third lens; a fourth lens; a fifth lens; and a sixth lens,arranged in this order from an object side to an image plane side,wherein said fifth lens is formed in a shape so that a surface thereofon the object side and a surface thereof on the image plane side havenegative curvature radii, said sixth lens is formed in a shape so that asurface thereof on the object side and a surface thereof on the imageplane side have positive curvature radii, and said fourth lens has anAbbe's number νd4, said fifth lens has an Abbe's number νd5, and saidsixth lens has an Abbe's number νd6 so that the following conditionalexpressions are satisfied:45<νd4<7520<νd5<4045<νd6<75.
 2. The imaging lens according to claim 1, wherein said firstlens has an Abbe's number νd1, said second lens has an Abbe's numberνd2, and said third lens has an Abbe's number νd3 so that the followingconditional expressions are satisfied:45<νd1<7520<νd2<4045<νd3<75.
 3. The imaging lens according to claim 1, wherein said fifthlens and said sixth lens have a composite focal length f56 so that thefollowing conditional expression is satisfied:−1.0<f56/f<−0.3 where f is a focal length of a whole lens system.
 4. Theimaging lens according to claim 1, wherein said first lens and saidsecond lens have a composite focal length f12 so that the followingconditional expression is satisfied:1.1<f12/f<3.0 where f is a focal length of a whole lens system.
 5. Theimaging lens according to claim 1, wherein said first lens has a focallength f1 and said second lens has a focal length f2 so that thefollowing conditional expression is satisfied:−0.8<f1/f2<−0.3.
 6. The imaging lens according to claim 1, wherein saidthird lens has a focal length f3 so that the following conditionalexpression is satisfied:1.0<f3/f<5.0 where f is a focal length of a whole lens system.
 7. Theimaging lens according to claim 1, wherein said fourth lens has a focallength f4 and said fifth lens has a focal length f5 so that thefollowing conditional expression is satisfied:−0.4<f4/f5<−0.1.
 8. An imaging lens comprising: a first lens havingpositive refractive power; a second lens having negative refractivepower; a third lens; a fourth lens; a fifth lens; and a sixth lens,arranged in this order from an object side to an image plane side,wherein said first lens is formed in a shape so that a surface thereofon the object side and a surface thereof on the image plane side havepositive curvature radii, said third lens is formed in a shape so that asurface thereof on the image plane side has a negative curvature radius,said fifth lens is formed in a shape so that a surface thereof on theobject side and a surface thereof on the image plane side have negativecurvature radii, and said fourth lens has a focal length f4 and saidfifth lens has a focal length f5 so that the following conditionalexpression is satisfied:−0.4<f4/f5<−0.1.
 9. The imaging lens according to claim 8, wherein saidfirst lens has an Abbe's number νd1, said second lens has an Abbe'snumber νd2, said third lens has an Abbe's number νd3, said fourth lenshas an Abbe's number νd4, said fifth lens has an Abbe's number νd5, andsaid sixth lens has an Abbe's number νd6 so that the followingconditional expressions are satisfied:45<νd1<7520<νd2<4045<νd3<7545<νd4<7520<νd5<4045<νd6<75.
 10. The imaging lens according to claim 8, wherein said fifthlens and said sixth lens have a composite focal length f56 so that thefollowing conditional expression is satisfied:−1.0<f56/f<−0.3 where f is a focal length of a whole lens system. 11.The imaging lens according to claim 8, wherein said first lens and saidsecond lens have a composite focal length f12 so that the followingconditional expression is satisfied:1.1<f12/f<3.0 where f is a focal length of a whole lens system.
 12. Theimaging lens according to claim 8, wherein said first lens has a focallength f1 and said second lens has a focal length f2 so that thefollowing conditional expression is satisfied:−0.8<f1/f2<−0.3.
 13. The imaging lens according to claim 8, wherein saidthird lens has a focal length f3 so that the following conditionalexpression is satisfied:1.0<f3/f<5.0 where f is a focal length of a whole lens system.
 14. Animaging lens comprising: a first lens having positive refractive power;a second lens having negative refractive power; a third lens; a fourthlens; a fifth lens having negative refractive power; and a sixth lens,arranged in this order from an object side to an image plane side,wherein said first lens is formed in a shape so that a surface thereofon the object side and a surface thereof on the image plane side havepositive curvature radii, said third lens is formed in a shape so that asurface thereof on the image plane side has a negative curvature radius,and said fourth lens has a focal length f4 and said fifth lens has afocal length f5 so that the following conditional expression issatisfied:−0.4<f4/f5<−0.1.
 15. The imaging lens according to claim 14, whereinsaid first lens has an Abbe's number νd1, said second lens has an Abbe'snumber νd2, said third lens has an Abbe's number νd3, said fourth lenshas an Abbe's number νd4, said fifth lens has an Abbe's number νd5, andsaid sixth lens has an Abbe's number νd6 so that the followingconditional expressions are satisfied:45<νd1<7520<νd2<4045<νd3<7545<νd4<7520<νd5<4045<νd6<75.
 16. The imaging lens according to claim 14, wherein saidfifth lens and said sixth lens have a composite focal length f56 so thatthe following conditional expression is satisfied:−1.0<f56/f<−0.3 where f is a focal length of a whole lens system. 17.The imaging lens according to claim 14, wherein said first lens and saidsecond lens have a composite focal length f12 so that the followingconditional expression is satisfied:1.1<f12/f<3.0 where f is a focal length of a whole lens system.
 18. Theimaging lens according to claim 14, wherein said first lens has a focallength f1 and said second lens has a focal length f2 so that thefollowing conditional expression is satisfied:−0.8<f1/f2<−0.3.
 19. The imaging lens according to claim 14, whereinsaid third lens has a focal length f3 so that the following conditionalexpression is satisfied:1.0<f3/f<5.0 where f is a focal length of a whole lens system.